Europe Engulfed Rulebook p15 wrote:
6.95 NATIONAL CONSTRUCTION LIMITS:
c) The Soviets (only) may construct ‘Heavy Fortifications’. They
are non-replaceable and only two may ever be built. They may only
be built in the Home Resource Areas identified on the map (see
terrain key). They cost 10 WERPs each, and they must replace an
existing Field Fortification which was built in a prior turn. They
continue to provide fortification benefits (are not destroyed) until
their area falls under Axis control. They adhere to all other fortification rules.
Check the map key but this should be what they are referring too.